The present invention relates to methods and apparatus for controlling a fluid stream in a pneumatic assembly.
Modem analytical instrumentation, such as a gas or liquid chromatograph, often requires accurate control of a fluid stream. Such instruments can employ one or more fluid streams in respective flow paths, and an extensive and complex array of channels, tubing, and fittings that are necessary for controlling the fluid flow. In addition, there is often a need to sense certain characteristics of the fluid at different points in the flow paths, such as the pressure, flow rate, and temperature of the fluid. These needs are typically addressed by the attachment of different sensors to the flow path, further increasing the complexity and physical volume of the flow system.
Diffusion bonded planar manifold technology offers a solution that simplifies the flow system. By eliminating the connecting tubing and fittings between different components, diffusion bonded planar manifolds provide a flow system that is compact, easily-manufactured, and reliable. A further advantage of diffusion bonded planar manifolds is that multiple fluid-handling functional devices may be coordinated and assembled in a small volume. This advantage results from pneumatic channels which are integrated into the diffusion bonded planar manifold, and which provide the fluid flow paths. The diffusion bonded planar manifold is also quite compact and amenable to construction in a variety of shapes and configurations, helping to minimize the volume of the flow system. However, the different flow paths in a diffusion bonded planar manifold often have balanced, or different flow rates which need to be controlled precisely for optimal performance of the instrument.
Commonly used devices for restricting fluid flow rates include discrete flow restrictors and fine bore tubing. Flow restrictors (xe2x80x9cfritsxe2x80x9d) are made of powder metals that are pressed or sintered into various porosity and shapes to provide the required pneumatic resistance. FIG. 1 shows a schematic representation of a porous metal frit in a pipe. The fluid flow enters at a high pressure and leaves at a lower pressure due to the pressure drop created across the frit element.
Flow restrictors are provided in holders, which are usually installed with elastomer seals. Alternately, flow restrictors are provided in various geometries that can be pressed into an assembly. In either case, flow restrictors are a separate part and require machined features, seals and/or fastening hardware to install. It is difficult to integrate flow restrictors into a thin diffusion bonded planar manifold without external seals or fastening hardware.
Fine bore tubing is available with thicker walls to provide small internal diameters and therefore pneumatic resistance if a long enough length is used. Fluid pressure drops as a function of fluid velocity and properties, tubing diameter and length, and friction due to pipe finish, fittings, and diameter changes. However, fine bore tubing also requires fastening hardware to install, and the size of tubing is usually larger than all the other features on a diffusion bonded planar manifold.
An alternative method to restrict flow is to reduce channel sizes in a diffusion bonded planar manifold. Restrictance by channel width (diameter) on diffusion bonded manifold plates, however, is limited by the etching and plating dimensions and the raw stock thickness. To date, pneumatic channels in diffusion bonded planar manifolds have been enlarged in size to provide more flow, but not decreased in size to restrict flow, because metal sheets with very small diameter channels are difficult to handle and prone to plugging during the bonding process.
The present invention provides a method and an apparatus to restrict flow rate in a diffusion bonded planar manifold with multiple, adjacent pneumatic channels. A forming pen with a bit is pressed into a diffusion bonded planar manifold causing the pneumatic channel inside to collapse onto itself in a predictable and controllable way. The bit has a geometry with angles and radii chosen or designed so that the surface of the diffusion bonded plate is dimpled but not ripped during the pressing operation. Further, the geometry of the bit limits the deformed area so that distortion of adjacent pneumatic channels is minimized, and the integrity of the deformed pneumatic channel and adjacent pneumatic channels is maintained (e.g., the pneumatic channels do not leak). No external seals or extra parts are required.
In a preferred embodiment, the apparatus also provides holding devices for the forming pen and/or the diffusion bonded planar manifold, as well as a pressing device to better control the pressure applied to the forming pen. In yet another preferred embodiment, the diffusion bonded planar manifold is connected to a regulated air supply and a flow meter so that the flow rate in the targeted pneumatic channel is monitored while the forming pen is pressed into the diffusion bonded planar manifold at a specific orientation. The method and apparatus disclosed herein may also be utilized to restrict flow rate in other types of planar manifolds, so long as they have a material property that allows plastic deformation without tearing or too much elastic springback.
These and other advantages will become obvious to those skilled in the art upon review of the following description, the attached drawings and appended claims. Although a preferred embodiment of the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.